Index modified coating on polymer substrate

ABSTRACT

The invention includes the structure of a multilayer protective coating, which may have, among other properties, scratch resistance, UV absorption, and an effective refractive index matched to a polymer substrate such as polycarbonate. Each layer may contain multiple components consisting of organic and inorganic materials. The multilayer protective coating includes interleaved organic layers and inorganic layers. The organic layers may have 20% or more organic compounds such as SiO x C y H z . The inorganic layers may have 80% or more inorganic materials, such as SiO 2 , SiO x N y , and ZnO, or mixtures thereof. Each layer of the multilayer protective coating is a micro layer and may have a thickness of 5 angstroms or less in various embodiments. The multilayer protective coating may contain in the order of hundreds or thousands of micro layers, depending upon the design requirement of applications. In each micro layer, the components may have substantially continuous variations in concentration.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation, and claims the benefit, commonly assigned U.S. patent application Ser. No. 12/070,660, filed Feb. 20, 2008, now U.S. Pat. No. 7,993,733, entitled “Index Modified Coating on Polymer Substrate.” The entire disclosure of which is incorporated herein by reference for all purposes.

BACKGROUND OF THE INVENTION

Polycarbonate has been used extensively in the automotive industry as a replacement to glass components. Sun roofs, windows, and lens covers are the primary components presently being made of polycarbonate. However, polycarbonate is susceptible to scratching and degradation from UV light. Hence, there is a need for developing a highly transparent protective coating for both scratch resistance and UV light absorption.

A lacquer based coating can be sprayed on polycarbonate and then thermally cured to provide a hard coating that also blocks UV light. Such a coating is typically in the range of 2-10 μm thick. The cost associated with this lacquer based technology is so high that it limits applications.

Another coating method is to first form a soft UV blocking layer benzyl phenon on a polycarbonate substrate. Next, a hard organo-silicon coating can be formed on the coated polycarbonate substrate by using plasma enhanced chemical vapor deposition (PECVD). The two-layer coating is normally 2-10 μm thick. Such an organo-silicon coating provides a much harder coating than the lacquer based system. However, the UV blocking layer beneath the organo-silicon coating is degraded from UV light attack over time. As a result of consumption of UV absorbers, the organo-silicon coating will typically shrink or crack after about 3000 to 5000 hours, which will shorten the lifetime of the scratch resistance coating. Therefore, there is a remaining need in the art to provide methods and systems for developing an endurable and transparent protective coating for UV blocking and scratch resistance.

BRIEF SUMMARY OF THE INVENTION

Embodiments of the invention makes use of pulse modulation techniques in a microwave plasma chemical vapor deposition (MPCVD) process to form a protective coating on a polymer substrate such as polycarbonate. The coating may have a matched effective refractive index to the polymer substrate and is able to provide protection for both scratch resistance and UV absorption.

Embodiments of the invention include the structure of a multilayer protective coating, which may have, among other properties, scratch resistance, UV absorption, and an effective refractive index matched to a polymer substrate such as polycarbonate. Each layer may contain multiple components consisting of organic and inorganic materials. The multilayer protective coating includes interleaved organic layers and inorganic layers. The organic layers may have 60% or more organic polymers such as SiO_(x)C_(y)H_(z). The inorganic layers may have 60% or more inorganic materials, such as SiO₂, SiO_(x)N_(y), and ZnO, or mixtures thereof. Each layer of the multilayer protective coating is a micro layer and may have a thickness of 5 angstroms or less in various embodiments. The multilayer protective coating may contain in the order of hundreds or thousands of micro layers, depending upon the design requirement of applications. In each micro layer, the components may have substantially continuous variations in concentration.

Embodiments of the invention also include the structure of a single gradient layer protective coating, which may have, among other properties, scratch resistance, UV absorption, and an effective refractive index matched to a polymer substrate such as polycarbonate. The single gradient layer may include multiple components of inorganic and organic materials, such as SiO₂, SiO_(x)N_(y), ZnO, and organo polymer (e.g. SiO_(x)C_(y)H_(z)), or mixtures thereof. The multiple components may be mixed to have substantially continuous variations in concentrations of the components. The single gradient layer may comprise 60% or higher concentration of dense components such as SiO₂ near the top and may comprise 60% or higher concentration of organo-silicon near the substrate surface.

Embodiments of the invention still further pertain to pulse modulation techniques in MPCVD for forming a protective coating with new properties in a single plasma process. In one set of the embodiments, a substrate processing system is provided. A housing defines a processing chamber. A coaxial plasma line source is located inside the processing chamber. A substrate is disposed inside the processing chamber. A gas-delivery system is configured to introduce gases into the substrate processing chamber. A controller controls the plasma-generating system and the gas-delivery system. The controller may also use pulse modulation techniques to control plasma radical density.

In different embodiments of the invention, examples of the precursor gases comprise silicon-containing precursors such as hexamethyldisiloxane (HMDSO) and hexamethyldilazane (HMDS), and the like, zinc-containing precursors such as diethylzinc (DEZ), oxidizing precursors such as oxygen or ozone, and nitrogen-containing precursors such as ammonia (NH₃). The reaction of HMDSO and an oxidizing precursor can form SiO₂ and a hard cross-linked polymer SiO_(x)C_(y)H_(z). Moreover, the reaction of HMDS, a nitrogen-containing precursor and an oxidizing precursor can form SiO_(x)N_(y) and organo-silicon. Furthermore, the decomposition of DEZ in the presence of oxidizing precursors may form ZnO.

In another set of embodiments of the invention, examples of modulation techniques comprise pulse width modulation and pulse amplitude modulation. Other modulation techniques comprise frequency modulation, pulse position modulation, and the like. By using the modulation techniques, the microwave power can be turned on and off to control the total radical density to alter the chemical composition of the layers formed on a polymer substrate. The layers may be a single gradient layer or a multilayer with varying properties in the micro layers.

Additional embodiments and features are set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the specification or may be learned by the practice of the invention. A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic cross-sectional drawing of a multilayer protective coating on a polymer substrate.

FIGS. 1B-1C are schematic cross-sectional drawings of a detailed gradient structure of layers 104, 106 of the multilayer protective coating.

FIG. 1D is another schematic cross-sectional drawing of a multilayer protective coating on a polymer substrate.

FIG. 2 is a schematic cross-sectional drawing of a single gradient layer protective coating on a polymer substrate.

FIG. 3A shows an exemplary pulse width modulation over time.

FIG. 3B shows an exemplary single gradient layer formed by a microwave pulse width modulation over time to control radical density.

FIG. 3C shows an exemplary multilayer structure formed by a microwave pulse width modulation with interleaved layers containing organic polymer or inorganic component such as SiO₂, SiO_(x)N_(y) or ZnO.

FIG. 4A shows an exemplary pulse amplitude modulation over time.

FIG. 4B shows an exemplary single gradient layer formed by a microwave amplitude modulation over time to control radical density.

FIG. 4C shows a exemplary multilayer structure of interleaved organic polymer layer and inorganic layer such as SiO₂, SiO_(x)N_(y) or ZnO formed by a microwave amplitude modulation over time to control radical density.

FIG. 5 illustrates the effect of pulsing frequency on the light signal from plasma.

FIG. 6 is a flow chart depicting the process steps that may be included in forming protective coating on a substrate.

FIG. 7 is a simplified schematic representation of a coaxial plasma line MPCVD chamber for forming oxide layers and organic polymers on a substrate.

FIG. 8A shows a chemical structure of HMDSO.

FIGS. 8B-8C provide the FTIR spectra of polymeric SiO₂ film and ceramic SiO₂ from HMDSO/O₂ by microwave plasma assisted CVD.

FIGS. 8D-8E illustrate chemical structures of polymeric SiO₂ and ceramic SiO₂ from HMDSO/O₂ by microwave plasma assisted CVD.

FIGS. 9A-9B show a SEM image and XPS spectrum of deposited SiO₂ thin film in accordance with the present invention.

FIGS. 10A-10C provide the XPS depth profile of deposited film SiO₂ in accordance with the present invention, the refractive index and optical transmission of a 5 μm thickness film.

DETAILED DESCRIPTION OF THE INVENTION 1. Overview of Microwave Plasma CVD Technology

Conventional thermal CVD processes supply reactive gases to a substrate surface where heat-induced chemical reactions take place to produce a desired film. Precursors contain one or more of the atoms or molecules necessary to form films of organic or inorganic material with desired properties. At low temperature or high vacuum condition, the reaction rate is below the gas arriving rate, so that film uniformity depends on temperature uniformity across the substrate and from substrate to substrate. Deposition rate is also controlled by temperature in conventional thermal CVD.

Plasma-enhanced CVD (PECVD) techniques promote excitation and/or dissociation of the reactant gases by application of radio-frequency (RF) energy to a reaction zone near the substrate surface, thereby creating a plasma. The high reactivity of the species in the plasma reduces the energy required for a chemical reaction to take place, and thus lowers the temperature required for such CVD processes when compared with conventional thermal CVD processes. These advantages may be further exploited by high density plasma CVD (HDPCVD) techniques, in which a dense plasma is formed at low vacuum pressure so that plasma species are even more reactive.

Furthermore, microwave-plasma assisted CVD (MPCVD) has been developed to achieve higher plasma densities (e.g. 10¹¹ ions/cm³) and higher deposition rate, as a result of improved power coupling and absorption at 2.45 GHz when compared to typical radio frequency (RF) coupled plasma sources at 13.56 MHz. The main drawback of the RF plasma is that a large portion of the input power is dropped across the plasma sheath (dark space). By using microwave plasma, a narrow plasma sheath is formed and more power would be absorbed by the plasma for creation of radical and ion species, which would increase the plasma density and reduce collision broadening of the ion energy distribution to achieve a narrow energy distribution.

Microwave plasma also has other advantages such as lower ion energies with a narrow energy distribution. For instance, microwave plasma would have low ion energy of 1-25 eV, which would lead to lower damage when compared to RF plasma. In contrast, standard planar discharge would result in high ion energy of 100 eV with a broader distribution in ion energy, which would lead to higher damage, as the ion energy exceeds the binding energy for most materials of interest. This ultimately inhibits the formation of high quality crystalline thin films through introduction of intrinsic defects. With low ion energy and narrow energy distribution, microwave plasma would help in surface modification and improve coating properties.

In addition, lower substrate temperature (e.g. lower than 200° C., for instance at 100° C.) is achieved in MPCVD, as increased plasma density at lower ion energy would help reduce defect density of films. Such a lower temperature would allow better microcrystalline growth in kinetically limited conditions. Also, it may require pressure greater than 50 mtorr to maintain self-sustained discharge for PECVD, as plasma becomes unstable at pressure lower than 50 mtorr. The microwave plasma technology allows the pressure to range from 10⁻⁶ torr to 1 atmospheric pressure.

Furthermore, there may be a reduction in impurities of the deposited coating, as there is no internal metallic electrodes for microwave discharges and thus the contamination of plasma and deposited films may be reduced.

In the past, the main drawback associated with microwave source technology in the vacuum coating industry was the scale up from small wafer processing to very large area processing. Recent advances in microwave reactor design have been able to overcome these problems. Arrays of new coaxial plasma line sources have been developed to deposit ultra large area coating (greater than 1 m²) at high deposition rate to form dense and thick films (e.g. 5-10 μm thick).

An advanced pulsing technique has been developed to control the microwave power for generating plasma, and thus to control the plasma density and plasma temperature. One of the advantages of the advanced pulsing technique is to minimize the thermal load disposed over substrate. This feature is very important when the substrate has low melting point or low glass transition temperature, such as polymer substrate. The advanced pulsing technique allows high power pulsing into plasma with off times in between pulses, which reduces the need for continuous heating of the substrate. Radical density may be controlled by process conditions including power level, pulse frequency, and duty cycle of the source. The radical density may be preferably affected by the average and peak power level into the plasma discharge. Film properties may be varied by changing radical species and radical density. To promote good adhesion to the substrate, composition of the organic film should be finely controlled, preferably in the form of a gradient across the film thickness. This desired gradient structure may be achieved by using the advanced pulse modulation technique in MPCVD.

2. Exemplary Multilayer Protective Coating

Zinc oxide is a stable inorganic crystal. It is commonly known for use as one of the most powerful whitening pigments. Zinc oxide is also found in sunscreen for UV light absorption, and is more stable than organic UV absorbers. The challenge of using zinc oxide for a highly transparent UV protective coating is the high refractive index of zinc oxide which is approximately 1.9-2.0. The effective refractive index of the protective coating may be reduced by alloying zinc oxide with other components of lower refractive index to match with substrate material (e.g. polycarbonate having a refractive index of approximately 1.6).

FIG. 1A shows a multilayer protective coating 108 on substrate 100. Layer 104 contains 60% or more inorganic components. Layer 106 contains 60% or more organic components. The two layers 104, 106 are interleaved. Each of the micro layers 104, 106 may be as thin as 5 Å or less. The multilayer protective coating 108 may contain in the order of hundreds or thousands of micro layers 104, 106. The substrate 100 comprises a polymer such as polycarbonate. The multilayer protective coating 108 may provide protection for scratch resistance, UV light absorption and an effective refractive index matched to the substrate 100.

FIG. 1B illustrates more details about micro layer 106. Three components are shown in FIG. 1B, where component C 116 comprises inorganic material, such as SiO₂, SiO_(x)N_(y). Component B 114 comprise also inorganic material, such as ZnO. Component A 112 comprises organic material, such as organo-silicon polymer (e.g. SiO_(x)C_(y)H_(z)). Note that the concentration of components B 114 and C 116 are higher than component A 112 in the micro layer 106. In general, the micro layer 106 may comprise more than 3 components as long as the total concentration of all inorganic components exceed 60%. FIG. 1B further demonstrates that there may be a substantial variation in concentration of the components in the micro layer 106.

FIG. 1C illustrates more details about micro layer 104. Similar to FIG. 1B, micro layer 104 comprises three components. However, micro layer 104 contains more component A 112 than components B 114 and C 116, where component A 112 comprises organic material, components B 114 and C 116 comprise inorganic materials. In general, the micro layer 104 may comprise more than 3 components as long as the organic components exceed 60%. FIG. 1C further demonstrates that there may be a substantial variation in concentration of the components in the micro layer 104.

Another embodiment of a multilayer protective coating 128 is illustrated in FIG. 1D. Conversely from FIG. 1 a, micro layer 124 is first disposed over a polymer substrate 120. Micro layer 126 is then disposed over micro layer 124. The micro layers 124 and 126 are interleaved to form the multilayer protective coating 128.

3. Exemplary Single Layer Protective Coating

FIG. 2 shows a single layer protective coating 208 on substrate 200. The single layer protective coating 208 comprises three components. Component A 202 contains organic material (e.g. SiO_(x)C_(y)H_(z)). Component B 204 contains inorganic material, such as ZnO which has UV blocking capability and higher refractive index of 1.9-2.0. Component C 206 contains inorganic material, such as SiO₂, SiO_(x)N_(y) which have good scratch resistance. The substrate 200 comprises polymer such as polycarbonate. FIG. 2 also demonstrates that there may be a substantial variation in concentration of the three components in the single layer protective coating 208. Generally, the single layer protective coating 208 may contain any number of components. Such components may be mixed so that substantial variations may exist in concentrations of all components. The single protective layer coating 208 may provide protection for scratch resistance, UV light absorption, and an effective index matched to the substrate 200.

4. Exemplary Power Modulation

There are many modulation technologies that control the radical densities and thus the chemical composition of deposited films. FIG. 3A shows a pulse width modulation (PWM) where the pulse amplitude remains constant, but the pulse width changes. FIG. 3A illustrates that when the pulse width is large, microwave power density is high so that a layer comprises high concentration of dense inorganic material such as SiO₂, SiO_(x)N_(y) and ZnO or mixtures thereof. When the pulse width is small, microwave power density is lower so that a layer comprises lower concentration of inorganic material such as SiO₂, SiO_(x)N_(y), and ZnO or mixtures thereof, but higher concentration of organo-silicon such as SiO_(x)C_(y)H_(z).

FIG. 3B illustrates a multilayer structure 308 formed on a substrate 300 by using a pulse width modulation. Note that a first layer 302 disposed over the substrate 300 comprises mainly SiO₂, a second layer 304 disposed over the first layer 302 comprises preferably organic polymer, and a third layer 306 disposed over the second layer 304 comprises mainly SiO₂ again.

FIG. 3C illustrates a single layer 314 formed on a substrate 300 by using a pulse width modulation. Again, when a pulse width is large, power density is high so that higher concentration of dense inorganic material may be formed. Note that the single layer 314 may comprise components 310 and 312 with substantially continuous variations in concentrations, where component 310 may comprise organo-silicon polymer such as SiO_(x)C_(y)H_(z), and component 312 may comprise dense inorganic material such as SiO₂. Furthermore, the single layer 314 may have higher concentration of the component 310 near the substrate, but higher concentration of component 312 near the top surface of the single layer 314.

FIG. 4A shows a pulse amplitude modulation (PAM) where the pulse width remains constant, the pulse amplitude changes. Similarly, when the pulse amplitude is large, microwave power density is high so that the film comprises high concentration of inorganic material, such as SiO₂, SiO_(x)N_(y), and ZnO, mixtures thereof. When the pulse amplitude is small, microwave power density is low so that the film comprises low concentration of inorganic material and high concentration of organic polymer such as SiO_(x)C_(y)H_(z).

FIG. 4B illustrates a multilayer structure 408 formed on a substrate 400 by using a pulse amplitude modulation. Note that a first layer 402 disposed over the substrate 400 comprises mainly SiO₂, a second layer 404 disposed over the first layer 402 comprises preferably organic polymer, and a third layer 406 disposed over the second layer 404 comprises mainly SiO₂ again.

FIG. 4C illustrates a single layer 414 formed on a substrate 400 by using a pulse amplitude modulation. Note that the single layer 414 may comprise components 410 and 412 with substantially continuous variations in concentrations, where component 410 may comprise organo-silicon polymer such as SiO_(x)C_(y)H_(z), and component 412 may comprise dense inorganic material such as SiO₂. Furthermore, the single layer 414 may have higher concentration of the component 410 near the substrate, but higher concentration of component 412 near the top surface of the single layer 414.

Pulsing frequency may affect the microwave pulse power into plasma. FIG. 5 shows the frequency effect of the microwave pulse power 504 on the light signal of plasma 502. The light signal of plasma 502 reflects the average radical concentration. As shown in FIG. 5, at low pulsing frequency such as 10 Hz, in the event that all radicals are consumed, the light signal from plasma 502 decreases and extinguishes before the next power pulse comes in. As pulsing frequency increases to higher frequency such as 10,000 Hz, the average radical concentration is higher above the baseline 506 and becomes more stable.

One of the benefits for using a pulse modulation technique is that a single gradient protective coating or a multilayer protective coating may be formed by changing the microwave power over time without changing major processing parameters such as process pressure, flow rate and the like.

It should be understood that the simplified PWM and PAM are presented for illustrative purposes only. One of the ordinary skill in the art could implement any other modulation technique, including but not limited to frequency modulation, amplitude modulation of any waveform, and pulse position modulation.

5. Exemplary Deposition Process

For purposes of illustration, FIG. 6 provides a flow diagram of a process that may be used to form a protective coating on a polymer substrate. The process begins with the substrate being loaded into a processing chamber as indicated at block 600. The substrate is then heated at block 602 to a low temperature (e.g. about 200° C. or less). Film deposition is initiated by flowing reactive precursor gases and carrier gases to the processing chamber at blocks 604, 606, 608, 610 and 612.

For deposition of SiO₂, such precursor gases may include a silicon-containing precursor such as hexamthyldisiloxane (HMDSO) and oxidizing precursor such as O₂. For deposition of SiO_(x)N_(y), such precursor gases may include a silicon-containing precursor such as hexmethyldislanzane (HMDS), a nitrogen-containing precursor such as ammonia (NH₃), and an oxidizing precursor. For deposition of ZnO, such precursor gases may include a zinc-containing precursor such as diethylzinc (DEZ), and an oxidizing precursor such as oxygen (O₂), ozone (O₃) or mixtures thereof. The reactive precursors may flow through separate lines to prevent them from reacting prematurely before reaching the substrate. Alternatively, the reactive precursors may be mixed to flow through the same line.

The carrier gases may act as a sputtering agent. For example, the carrier gas may be provided with a flow of H₂ or with a flow of inert gas, including a flow of He or even a flow of a heavier inert gas such as Ar. The level of sputtering provided by the different carrier gases is inversely related to their atomic mass. Flow may sometimes be provided of multiple gases, such as by providing both a flow of H₂ and a flow of He, which mix in the processing chamber. Alternatively, multiple gases may sometimes be used to provide the carrier gases, such as when a flow of H₂/He is provided into the processing chamber.

As indicated at block 614, a plasma is formed from precursor gases by microwave at a frequency of 2.45 GHz. In some embodiments, the plasma may be a high-density plasma having an ion density that exceeds 10¹¹ ions/cm³. Also, in some instances the deposition characteristics may be affected by applying an electrical bias to the substrate. Application of such a bias causes the ionic species of the plasma to be attracted to the substrate, sometimes resulting in increased sputtering. The environment within the processing chamber may also be regulated in other ways in some embodiments, such as controlling the pressure within the processing chamber, controlling the flow rates of the precursor gases and where they enter the processing chamber, controlling the power used in generating the plasma, controlling the power used in biasing the substrate and the like. Under the conditions defined for processing a particular substrate, material is thus deposited over the substrate as indicated at block 618. After deposition is completed, the plasma is extinguished at block 620 and the substrate transferred out of the processing chamber 600.

6. Exemplary Deposition System

FIG. 7 is a simplified diagram of a coaxial microwave-assisted chemical vapor deposition (CVD) system 700 according to embodiments of the invention. Multiple-step processes can also be performed on a single substrate or wafer without removing the substrate from the chamber. The major components of the system include, among others, a processing chamber 724 that receives precursors from precursor gas line 704 and carrier gas line 706, a vacuum system 724, a coaxial microwave line source 726, a substrate 702, and a controller 732.

The coaxial microwave line source 726 includes, among others, an antenna 712, a microwave source 716 which inputs the microwave into the antenna 712, an outer envelope surrounding the antenna 712 made of dielectric material (e.g. quartz), which serves as a barrier between the vacuum pressure 708 and atmospheric pressure 714 inside the dielectric layer 710. The atmospheric pressure is needed for cooling the antenna 712. Electromagnetic waves are radiated into the chamber 724 through the dielectric layer 710 and plasma 718 may be formed over the surface of the dielectric material such as quartz. In a specific embodiment, the coaxial microwave line source 726 may be about 1 m long. An array of the line sources 626 may sometimes be used in the processing chamber 724.

The precursor gas line 704 may be located below the coaxial microwave line source 726 and above the substrate 702 which is near the bottom of the processing chamber 724. The carrier gas line may be located above the coaxial microwave source 726 and near the top of the processing chamber 724. Through the perforated holes 704 and 722, the precursor gases and carrier gases flow into the processing chamber 724. The precursor gases are vented toward the substrate 702 (as indicated by arrows 728), where they may be uniformly distributed radically across the substrate surface, typically in a laminar flow. After deposition is completed, exhaust gases exit the processing chamber 724 by using vacuum pump 724 through exhaust line 730.

The controller 732 controls activities and operating parameters of the deposition system, such as the timing, mixture of gases, chamber pressure, chamber temperature, pulse modulation, microwave power levels, and other parameters of a particular process.

While the above is a complete description of specific embodiments of the present invention, various modifications, variations and alternatives may be employed. Moreover, other techniques for varying the parameters of deposition could be employed in conjunction with the power modulation technique. Examples of the possible parameters to be varied include but are not limited to the temperature of deposition, the pressure of deposition, and the flow rate of precursors and carrier gases.

Having described several embodiments, it will be recognized by those of skill in the art that various modifications, alternative constructions, and equivalents may be used without departing from the spirit of the invention. Additionally, a number of well known processes and elements have not been described in order to avoid unnecessarily obscuring the present invention. Accordingly, the above description should not be taken as limiting the scope of the invention.

7. Exemplary Ceramic SiO₂ and Polymer-Like SiO₂ Films

A few analytical techniques are briefly described here, and may be used to characterize the films formed from the process in accordance with the present invention.

Fourier Transform Infrared spectroscopy (FTIR) is commonly used to identify organic compounds or polymers based upon a group of peaks detected. FIGS. 8A-8B show the FTIR spectra for two types of SiO₂ formed from precursors HMDSO and O₂ by using pulse modulation in MPCVD, i.e. ceramic SiO₂ film and polymer-like SiO₂ film. The chemical structure of HMDSO is provided in FIG. 8A. As shown in FIG. 8B, the FTIR peaks indicate the presence of CH bond, SiH bond, Si—CH₃ functional group, and Si—OH bond. With the detected peaks, the chemical is identified as SiO_(x)C_(y)H_(z). FIG. 8C shows the FTIR spectrum of SiO₂ where fewer peaks are observed than in FIG. 8B.

The inventors have performed a number of experiments to demonstrate that the coaxial microwave line source is suitable for depositing dense film at high deposition rate (e.g. >3 μm/min), such as amorphous and microcrystalline silicon, SiO₂, SiO_(x), SiN_(x), SiO_(x)N_(y), diamond and the like. The gradient films of SiO₂, SiO_(x), SiN_(x), SiO_(x)N_(y) may be formed by using the microwave pulsing modulation technology according to embodiments of the invention. The inventors have also demonstrated that highly scratch resistant SiN_(x) is successfully deposited on polycarbonate substrate without poor film adhesion problem.

FIG. 9 provides a scanning electron microscope (SEM) image and X-ray photoelectron spectroscopy (XPS) spectrum of a ceramic SiO₂ thin film. XPS is also called electron spectroscopy for chemical analysis (ESCA), and is a surface analysis technique. It can measure the elemental composition of the surface to a depth of about 50-100 Å. By sputtering, depth profiles of all detectable elements can be obtained to show the elemental composition variation in deposited film. FIG. 9B shows that the silicon and oxygen content remain approximately constant until the time of sputtering reaches 18,000. Then, the silicon content increases significantly while the oxygen content decreases sharply, and the carbon content increases. This indicates the presence of SiO₂ layer up to a sputtering time of 18,000 and the transition to a new layer near the sputtering time of 18,000. With the surface analysis technique described, the depth profile of any film formed in accordance to the present invention can be characterized, for example to show a gradient in elemental composition.

Referring now to FIG. 10A, the XPS depth profile indicates that the deposited layer is SiO₂. FIG. 10B illustrates that the deposited dielectric layer has a refractive index of about 1.43-1.46 depending upon wavelength. FIG. 10C shows that the deposited SiO₂ layer has an optical transmission between 80% and 90% varying with wavelength. This technique can be used to characterize the coating and to verify if the coating has the desired optical transparency, for example, the protective coating comprising ZnO with at least one of SiO₂ and SiO_(x)N_(y) and having a matched effective refractive index to polycarbonate substrate. Such a protective coating may have scratch resistance, UV light resistance and may be optically transparent.

Furthermore, other techniques may be used in material characterization. For example, X-ray diffraction technique can be used for verifying the structure of deposited film such as the diamond structure. Raman spectroscopy can be used to distinguish amorphous silicon and microcrystalline silicon.

Those of ordinary skill in the art will realize that specific parameters can vary for different processing chambers and different processing conditions, without departing from the spirit of the invention. Other variations such as structure of the protective coating, material components, reactive precursor gases, will also be apparent to persons of skill in the art. These equivalents and alternative are intended to be included within the scope of the present invention. Therefore, the scope of this invention should not be limited to the embodiments described, but should instead be defined by the following claims. 

What is claimed is:
 1. A method for forming a multilayer protective coating comprising: providing a continuous flow of a group of precursors to a chamber, wherein the precursors comprise at least one silicon-containing precursor, a zinc-containing precursor, an oxidizing precursor, and a nitrogen-containing precursor; providing a modulated microwave plasma source for controlling plasma radical concentration so that material composition may be varied by changing microwave power over time, and causing reactions among the precursors to form oxides and organic polymers to produce a coating comprising: a plurality of organic layers having at least 60% organic content; and a plurality of inorganic layers having at least 60% inorganic content that are interleaved with the organic layers.
 2. The method for forming a multilayer protective coating according to claim 1, the process comprising: causing a reaction between a first silicon-containing precursor and the oxidizing precursor to form silicon oxide components; causing a reaction among a second silicon-containing precursor, the nitrogen-containing precursor, and the oxidizing precursor to form silicon oxynitride components; and causing a reaction between the zinc-containing precursor and the oxidizing precursor to form zinc oxide components.
 3. The method for forming a multilayer protective coating according to claim 1 further comprising: modulating a microwave pulse amplitude using a waveform; depositing an organic material using a first microwave power; and depositing an inorganic material using a second microwave power greater than the first microwave power.
 4. The method for forming a multilayer protective coating according to claim 1 further comprising: modulating a microwave pulse width using a waveform; depositing an organic material during a time when the microwave power pulse is off; and depositing an inorganic material during a time when the microwave power pulse is on. 